During June 6-10, 1991, as part of an ongoing study to
identify the sources of Aedes albopictus bloodmeals, researchers
from the University of Notre Dame collected mosquitoes in and
around a tire dump in Polk County, Florida. The collections were
made with a Nasci aspirator and yielded 9393 Ae. albopictus
mosquitoes that were sent to CDC in December 1991; 9350 were
tested, in 96 pools, for virus isolation by plaque assay in Vero
cell culture. Forty-three blood-fed specimens were tested
separately for bloodmeal identification. The specimens tested for
virus yielded 14 virus strains identified as eastern equine
encephalitis (EEE) virus by indirect fluorescent antibody test
using a panel of alphavirus monoclonal antibodies including EEE
virus complex-specific (1B1C-4) and North American EEE
virus-specific (1B5C-3) monoclonal antibodies (1). The virus
strains were reisolated from the original mosquito pools by
intracranial inoculation into 1- to 3-day-old suckling mice. Two
representative isolates were confirmed as EEE virus by
plaque-reduction neutralization test. Results of the bloodmeal
identification were 31% bovine, 24% unidentified mammal, 19% deer,
14% human, 7% raccoon, 5% rabbit and 2% passeriform birds.

During June 1991, Ae. albopictus mosquitoes were collected in
two other Florida counties and tested for virus: 100 specimens, in
two pools, from Gilchrist County and 430 specimens, in six pools,
from Marion County were negative for EEE.

The tire dump, which has been closed since 1988, contains
approximately 8 million tires. Scientists from the University of
Notre Dame and the Florida Medical Entomology Laboratory in Vero
Beach have been sampling Ae. albopictus populations at the dump
since 1989.

Editorial Note

Editorial Note: This is the first documented isolation of EEE virus
from Ae. albopictus collected in the United States. In the United
States, EEE is the rarest of the mosquitoborne arboviral
encephalitides but has a human case-fatality rate of approximately
30% (2). The virus is maintained in fresh-water-swamp habitats in
an enzootic cycle involving mosquitoes, principally Culiseta
melanura, and a variety of bird species. During 1991, heavy spring
rains in northern Florida led to exceptionally large populations of
Cs. melanura as well as mosquito species that serve as epizootic
vectors (3,4). Consequently, Florida experienced early, widespread
EEE virus activity with 70 equine cases reported by the beginning
of July, the highest reported in a season by that time (3). Polk
County reported four confirmed EEE cases in equines, three of them
with onset in May and June; date of onset for the other case is
unknown (CDC, unpublished data). Therefore, epizootic transmission
of EEE virus was occurring in Polk County during the same period
that infected Ae. albopictus mosquitoes were collected at the tire
dump. In addition, the Florida Department of Health and
Rehabilitative Services (HRS) confirmed five human cases of EEE
among elderly residents during this period (3), but all were from
an area approximately 125 miles north of Polk County.

From 1986 through 1991, Ae. albopictus extended its range from
one to 61 of Florida's 67 counties (5) and is widely distributed in
Polk County; it has been collected from 47 of 148 CDC light-trap
locations (M. Mahler, Polk County Environmental Services, personal
communication, 1991). In addition, results from vector-competence
studies showed that a strain of Ae. albopictus mosquitoes from
Houston became infected with EEE virus after feeding on viremic
chicks, and 57% of those mosquitoes refeeding transmitted the virus
15 days postinfection (6). This information, along with findings
reported here regarding virus isolations from field-collected
specimens and with the opportunistic feeding habits of Ae.
albopictus, suggest that Ae. albopictus may become an epizootic and
epidemic vector of EEE virus. Plans are under way by the Florida
HRS and CDC to improve surveillance for EEE in Polk County and to
initiate collaborative studies during the summer of 1992 to more
clearly define the role of Ae. albopictus in the EEE transmission
cycle. States with Ae. albopictus infestations and a history of EEE
should increase surveillance for both human and equine cases, as
well as for EEE virus activity among mosquitoes and birds.

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